Usually, a snowplow, a farm tractor or like work vehicle is structured so that an auger or the like is driven using power that is branched and output from an engine or a power transmission system.
For example, a snowplow comprises, as shown in FIG. 8, a controller 100, an engine chamber 101, an auger 102, a blower 103, a duct 104, and a crawler 105. Output from an output shaft 106 is branched into a PTO shaft 111 (implement system) for driving the auger 102 and a drive shaft 112 (traveling system) of the crawler 105 via pulleys 107 to 110. Rotational driving force is transmitted from the drive shaft 112 to an axle 114 via a transmission 113. A hydrostatic transmission is often used in such a work vehicle as a speed change gear.
However, if an auger or like implement system becomes overloaded for some reason while such a work vehicle is in working operation, the engine may stop. Even if the engine does not stop, the rotation becomes less than ideal for driving an implement, greatly reducing workability (work efficiency).
In the case of a snowplow, the auger 102 is smoothly rotated and snow can be removed when clearing fresh snow. However, when clearing spring snow, since the snow has a high moisture content, the duct 104 tends to become blocked with snow. This may result in an engine failure, because an excessive load is applied to the engine via the auger.
In prior art work vehicles provided with a hydrostatic transmission, a mechanical control means and an electronic control means are known for preventing the engine from stalling and for maintaining the engine speed at the most desirable level for operation.
A mechanical control means is disclosed in, for example, Japanese Unexamined Patent Publication No. 1998-184906. This control means comprises an overload prevention system that reduces overload by providing an elastic body between a variable displacement oil-hydraulic pump and a controller to control the variable displacement of the pump.
An electronic control means is disclosed in, for example, Japanese Unexamined Patent Publication No. 1997-79375. In this control means, an oil-pressure sensor is provided in an oil-hydraulic line in a hydrostatic transmission, and a swash plate in the oil-hydraulic pump is controlled by an electric motor using an electronic-control means based on electrical data regarding the oil pressure in the pump that is detected by the oil-pressure sensor.
However, in the mechanical control means disclosed in Japanese Unexamined Patent Publication No. 1998-184906, control becomes difficult due to the deterioration of the elastic body with the passage of time.
Furthermore, the electronic control means disclosed in Japanese Unexamined Patent Publication No. 1997-79375 requires a large actuator (i.e., electric motor, etc.), because the swash plate has to be driven against the oil pressure. Furthermore, it is necessary to release the interlock between the gearshift and the swash plate while placing the gearshift near the operator's hand in the traveling position that has been determined by the operator, and then to switch to the interlock between the swash plate and the control actuator, making the structure complicated.